The storm felt ’round the world

Data suggests that the breakup of a massive iceberg near Antarctica was …

You wouldn't guess it by their appearance—empty slabs of ice drifting slowly through frigid seas—but large icebergs sing with energy. Even the slightest oceanic swell, as it moves unevenly below the surface of icebergs that may be up to 100 kilometers across, will strain and flex the surface of the berg in a way that sets off internal tremors with a vibrational frequency of around a single Hz.

But oceanic swells of different energies travel through the oceans at different speeds. As a result, for events that generate a point source of waves, the change in frequency at which the iceberg is humming gives a rough estimate of how far away the source of the swell was. Using seismographs situated on icebergs near Antarctica, researchers were able to pick up the signatures of the Indonesian tsunami and track Typhoon Olaf as it made its way across the pacific.

The scientists who were crunching this data were in for a bit of a surprise when they calculated the distance on one storm: over 13,000 km, with a travel time of six days to reach the iceberg. Crunching through the meteoroligcal records, they found the culprit in the form of a large storm in the Gulf of Alaska. They found records in other locations, and could actually track the swells as they moved across the Pacific and towards their sensors. Although this event stood out, it was part of a larger trend. Looking through their data, they found that there was actually a large increase in seismic activity from December through March, a time when the waters near Antarctica are enjoying their period of summertime calm (the solar-powered seismographs shut down in the winter). It turns out that these are the peak months for both the Typhoon season and winter storms in the Arctic. Thus, much of the seismic activity in the Antarctic summer is generated thousands of kilometers away.

The distance of the Alaskan storm wasn't the only notable aspect of it—the timing was auspicious as well. Less than 12 hours after the swell arrived, the iceberg shattered into six major pieces. How could these weak swells break up an iceberg that was over 100 km long and 25 km across? Seismic stations in the Antarctic interior picked up the event, suggesting that the waves crunched the iceberg onto the earth at a shoal it had gotten stuck on. These results suggest that the dynamics of the oceanic ice sheets near Antarctica are influenced not only by the added warmth of summer, but also by of the actions of storms a full hemisphere away.